Interestingly, Obama has made some MAJOR changes that many people disagree with, and we're in the midst of a serious recession.
And his popularity number (by gallup, the most reliable indicator for the last 4 decades) still have him at 53 to 55%. So he'd still win the election today.
Common sense (and history!) says a president will do all the controversial and unpopular stuff early in his presidency, then go moderate later in his term to get re-elected. Once obama starts doing popular stuff, such declaring a win in afghanistan, killing bin laden, or fixing unemployment, that 53% approval will suddenly be 60 or 70% again.
And don't forget the repubs (historically) go far-right at times like these. They'll run a Palin in 2012 based on her ability to win that repub base, but the moderates aren't going to buy the bible-thumping, "world is only 5000 years old" stuff.